The invention concerns a nuclear magnetic resonance (NMR) tomography system with a cryostat having a horizontal room temperature bore with a diameter d between 20 cm and 50 cm, preferentially between 20 cm and 40 cm. The system has a vessel filled with liquid helium and a superconducting solenoid coil, in intimate thermal contact with the liquid helium and with a horizontal axis (r=0), for the production, within the room temperature bore, of a static homogeneous magnetic field in a homogeneity volume. The homogeneity volume has a center located at a separation l.sub.m from an axial end (z=0) of the room temperature bore.
An NMR tomography system of this kind is, for example, known from the company brochure BIOSPEC-Series of the company Bruker Medizintechnik GmbH, D-76287 Rheinstetten, of March 1992.
The NMR tomography magnet systems which are known in the art serve, among other things, for in-vivo spectroscopy and image slice production in human medicine as well as for the investigation of small animals for purposes of research. Whereas the apparatuses with large magnet bore diameters in the range of 80-100 cm are utilized for full body investigations of patients, systems with magnetic bore diameters between 20 and 60 cm are normally primarily used for research purposes.
The apparatuses known in the art are poorly suited for partial body investigations of human extremities, for example, of a hand or a foot or even for the imaging of a human knee, since, with full body tomographs, the entire patient must be inserted and thereby, in general, signals from non-interesting regions of the body are simultaneously recorded during partial body investigations. Although an individual arm can be investigated approximately up to the elbow with smaller magnets in special cases, it is, however, not possible to undertake a partial body investigation of the human leg, in particular, in the knee region with apparatuses which are known in the art due to their geometry, since even a healthy patient would not be able to introduce the other leg which is to be investigated at right angles and horizontally extended into the narrow magnet bore. This is certainly not possible with sick or injured patients.
A superconducting magnet system for partial body tomography is known from DE-40 10 032 A1 with which an uninhibited access to the measuring volume of the system is easily possible from above due to the C-shaped cross section geometry of the corresponding cryostat to, in fact, substantially simplifies NMR investigation of the patient. This is, however, attained in the system known in the art at the expense of a very complicated cryostat construction which is costly to produce whereby, in addition, the forces with which the magnet coils pull each other which, for this type of magnet, are on the order of magnitude of several tons must be contained unsymmetrically which, due to the required safety standards, adds additional expense to the static construction of the system. In addition, a solenoid coil is not utilized in the C-shaped magnets known in the art rather a system of partially opposing coils so that, for equal volumes, a substantially lower maximum field strength can be produced than with a solenoid coil system.
A superconducting NMR magnet having a "temple-shaped" construction is known in the art from EP 0 350 267 A1 which allows for a comfortable sidewards access to the patient to be examined but which, however, requires a very complicated cryostat construction compared to conventional hollow cylindrical constructions. Due to the special coil shape of the temple-shaped magnet known in the art there is, on the other hand and in contrast to a solenoid magnet, an extremely poor utilization of field strength and therefore a substantially lower resulting NMR image resolution. Fine structures, which are particularly important for an orthopedist, can therefore not be sufficiently accurately seen.
Known in the art from U.S. Pat. No. 5,117,118 is, finally, a quasi open magnet configuration for NMR imaging, the so-called inside-out-magnet, with which the magnet coil exhibits a very large diameter and an extremely small depth and the investigational volume lies just outside of the magnet coil. However, a magnet system of this type has, in contrast to solenoid coils, the disadvantage of a strong outer region irradiated stray field, a substantially lower field homogeneity, a substantially smaller homogeneity region and a substantially lower maximum produceable field strength so that the resolution of the NMR images is much lower than with the above mentioned conventional magnet systems having solenoid coils such as those known in the art by the cited BIOSPEC-product line of the company Bruker Medizintechnik GmbH. For a given length of superconducting wire material, which contributes essentially to the price of such a magnet system, an inside-out-magnet coil is much less economical.
It is therefore the purpose of the present invention to create an NMR tomography magnet system for partial body investigations of human extremities which is easy and compact in construction as well as economical to produce and which allows for high resolution while thereby producing a relatively low stray field and with which the patient can be investigated in a comfortable body position.